1. Field of the Invention
The present invention pertains to Langmuir-Blodgett films and more particularly to those capable of being applied in nonlinear optics.
The Langmuir-Blodgett method makes it possible to obtain very thin (10 to 40 angstroms) organized monolayers on the surface of a liquid, to transfer these monolayers and to superimpose them on a substrate. The possibilities for the application of these films are derived from the following characteristics: the simplicity of the preparation method, high precision of thicknesses obtained (a few angstroms), the orientation of the molecules and adjustable physical properties.
2. Description of the Prior Art
In nonlinear optics, the physical property that is sought to be used is a second order or third order nonlinear optical susceptibility. Thus, for the generation of second harmonic waves, the useful Langmuir-Blodgett films are those having a non-centrosymmetrical structure. Structures of this type are already known in the prior art. They are classified according to the orientation imposed on the molecules of the different monolayers that form the films. It is thus possible to prepare films formed of X type or Z type layers, i.e. where all the molecules are placed in the same direction. It is also possible to make films formed of A, B, A, B type layers, i.e. where some layers are formed of A molecules and others are formed of B molecules, with the various layers alternating. The noteworthy fact about these structures is that they result in films for which the axis of maximum hyperpolarizability is perpendicular to the substrate. The result of this is that a light wave arriving in a direction perpendicular to the substrate (the associated electrical field being therefore parallel to the substrate) has no interaction with these films. For example, in one experiment for the generation of second harmonics, the amplitude of the wave generated at 2 .omega. in a direction perpendicular to the plane of the substrate is nil. Now, in many applications of these Langmuir-Blodgett films, it is necessary to obtain the generation of the second harmonic in a direction perpendicular to the substrate.
To remove these disadvantages, the present invention proposes films, prepared by the Langmuir-Blodgett method, comprising at least one layer of a mixture of molecules, the mixture being chosen so as to obtain the desired properties. In mixing molecules of different natures, it becomes possible for these molecules to adopt a configuration and arrangement different from those they would have had if used alone.